With the current fleet of operational launch vehicles namely PSLV and GSLV, India can meet the requirements of launching communication satellites up to 2.2 tons to GTO. With the introduction of GSLV Mk-III, the payload capability has been doubled which will meet the national requirement of launching communication satellites.

GSLV Mk-III is a three stage vehicle designed for catering the need of carrying heavier communication satellite to GTO. The vehicle lift off mass is 640 ton with overall height of 43.498 m and core diameter of 4 m. The vehicle has two Solid Strap-on motors - S200, a core liquid booster stage - L110, and a cryogenic upper stage - C25. To accommodate heavier payloads, 5 m diameter Ogive Payload Fairing is employed.

S200 strap on motor is a 3.2 m diameter solid motor. It is made up of 3 motor segments and has a flex nozzle control system. To reduce the disturbance moment due to differential thrust between the two S200 strap on motors in flight, the two motors are processed as a pair during the motor segments casting. L110 liquid Stage, works on two clustered Vikas engines which are being used in PSLV & GSLV. It has 110 tons of propellant loading and each of the twin engines produces 80 tons of thrust. The upper stage in GSLV MK-III vehicle is cryogenic stage with 28 tonnes of propellant loading and designated as C25. The C25 Stage is powered with a 20 ton thrust (nominal) engine working on Gas Generator (GG) cycle. C25 stage is a high performance cryo stage carrying a propellant combination of liquid Hydrogen stored at 20 K and liquid Oxygen stored at 77 K. C25 Engine & stage were developed and validated through a series of 200 tests which included qualification tests through a step by step process of component level test, engine level test for a duration of 800 s and stage level test for a duration of 640s equal to its flight time. Unlike the Cryogenic Upper Stage (CUS) of GSLV, which was based on the Russian engine design, C25 is entirely indigenous starting from configuration, design, development and qualification strategies. The entire test programme of integrated C25 Engine & stage were done in fast track mode and completed in a short time frame of two years using limited number of hardware and optimal sequencing of tests. During the maiden flight of GSLV Mk-III on June, 5 2017 the vehicle carried the GSAT-19 satellite onboard, weighing 3,136 kg, to the targeted GTO of 170km by 36,000km.

The vehicle lifted off from the launch pad upon the simultaneous ignition of both S200 motors. L110 core stage ignited during S200 thrusting phase itself at 112.66 s after lift off to augment the thrust of the vehicle and continued to function beyond the separation of two solid strap-ons which occurred at 140.84s from lift off. After nearly 206 s of firing, L110 stage separated followed by the ignition of C25 cryogenic stage. C25 stage operated for around 625 s duration and once the required orbital conditions were achieved, the cryogenic stage shut off the engine. Then the GSAT-19 satellite was injected into GTO. The performance of the two paired S200 motors were as predicted during flight and the differential thrust between the motors was benign. L110 stage performed exactly as per prediction & the performance of the twin clustered Vikas engines was identical. It is to be noted that even though GSLV Mk-III D1was the second flight as far as the S200 and L110 are concerned, for the newly developed C25 stage this was the maiden flight. The performance of the C25 stage in this maiden flight was as per prediction, which establishes the capability of ISRO to predict the flight performance in advance.

The experimental flight of GSLV Mk-III (LVM3-X) was undertaken on December 18, 2014 with S200 and L110 stages to demonstrate the atmospheric regime of the flight and unique features in GSLV Mk-III, compared to other launch vehicles, including the differential thrust between the two S200 solid strap-ons.

Based on the flight data analysis of LVM3-X mission, suitable improvements have been incorporated in GSLV Mk-III vehicle configuration, mainly on aerodynamic shaping, which included Ogive shaped Payload Fairings, Slanted Nose Cones for S200, aero shaping of cowlings & shrouds and closed Inter-Tank Structure for C25 stage. This has helped in improving the vehicle robustness, with better aerodynamic margins and reduced overall acoustic levels. Modification in S200 motor Head End Segment grain configuration was also done to reduce the dynamic pressure during flight. All these changes were qualified through ground tests as well as detailed characterisation tests and were incorporated in GSLV MK-III D1 vehicle.

With the successful completion of the first development flight, ISRO now gears up for the second development flight - GSLV Mk-III D2 with augmented payload capability so that the first operational flight of GSLV MK-III will carry around 4000 kg payload to GTO. The strategies/technologies for achieving the above capabilities are identified and ISRO is working towards perfecting these technologies before implementation in the flight.

Success of the first development flight of GSLV Mk III on June 05, 2017 is indeed a rare feat.